1. Field of the Invention
The present invention relates to an image forming apparatus and a drive control method for a liquid ejection head, and particularly relates to an image forming apparatus that forms images using a liquid ejection head having pressure generating elements corresponding to multiple ejection ports (nozzles), and to a drive control technique for a liquid ejection head suitable for this apparatus.
2. Description of the Related Art
Generally, in inkjet recording apparatuses (inkjet printers), printing is performed by ejecting ink droplets from the nozzles of a recording head at specific timings on the basis of dot pattern data (also referred to as “dot data” or “print data”) resulting from the development of image data for printing inputted from a host computer, and depositing and sticking these ink droplets onto recording paper or another such print recording medium.
A known example of a recording head system is a system that ejects ink droplets by varying the volume of a pressure chamber (pressure creating chamber) communicated with the nozzle opening. In this type of recording head, a diaphragm capable of elastic deformation in the out-of-plane direction is formed on part of the peripheral wall formed to partition the pressure chamber, and the volume of the pressure chamber is varied by vibrating the diaphragm with a pressure generating element, typified by a piezoelectric element.
Normally, a plurality of nozzle openings are formed in the recording head, and a pressure chamber and a piezoelectric element are provided for each nozzle opening. All of the piezoelectric elements are electrically connected in parallel between a common electric supply line and a ground line, and switching elements are electrically connected in series for the respective piezoelectric elements. Signals (driving waves) for driving the piezoelectric elements are generated by a driving wave generating circuit, and are selectively distributed and supplied to the piezoelectric elements via the electric supply line and the switching elements.
More specifically, when a specific switching element is selected and turned on according to the print data, a driving wave is applied to the piezoelectric element via the electric supply line, and an ink droplet is ejected from the specific nozzle opening corresponding to the piezoelectric element to which the driving wave is applied.
Inkjet recording apparatuses that use piezoelectric elements as described above usually have a common drive circuit system, in which one common driving wave resulting from a combination of a plurality of driving wave elements for ejecting a plurality of types of ink droplets with different ink volumes (for example, for a large dot, medium dot, and small dot) is provided, and one of the wave components necessary for each piezoelectric element is selectively applied by switching (see Japanese Patent Application Publication Nos. 2002-154207 and 2000-37867). This system has advantages in that there is no need to separately prepare a plurality of driving wave generating circuits for the respective piezoelectric elements, and that the number of high voltage and high precision analog circuits and the number of wires can be reduced, since the common driving wave is simultaneously applied to the plurality of piezoelectric elements.
Also, printers with an array system or a line system have recently been proposed with the object of increasing the printing speed, in which an extremely large number of nozzles are arranged and ink is simultaneously ejected from multiple nozzles to perform high-speed print recording. Array system or line system recording heads with multiple nozzles have problems in that if the common drive circuit system described above is applied as is, multiple piezoelectric elements are simultaneously driven by a driving wave output from the single drive circuit, which causes driving wave distortion due to heavy load fluctuations and causes unsatisfactory ejections, and results in image unevenness. Moreover, since the multiple piezoelectric elements are simultaneously driven, there is the possibility that a large electric current will instantaneously flow through the transistor that constitutes a power amplifier in the drive circuit, the electric current will exceed the driving capabilities of the transistor (Icmax: the maximum collector current), and the generated heat will exceed the allowable power dissipation of the transistor (Pcmax: the maximum collector power dissipation).
Increasing the size of the transistor used in the power amplifier and increasing the size of the radiator have been considered as solutions for this problem. However, the response of large transistors is slow and not enough for a driving wave with a shorter ejection cycle, which generally has a faster wave switching time, then the optimum power amplifier does not exist, and the radiator must be extremely large as a result.
Therefore, in conventional practice, configurations have been proposed in which the nozzles are driven by a plurality of drive circuits. For example, Japanese Patent Application Publication No. 6-127034 discloses a circuit dividing system in which the nozzles in the recording head are divided into groups, and the nozzles are driven using separate drive circuits for the respective groups. Dividing the load with a plurality of drive circuits in this manner makes it possible to reduce the load for each drive circuit, to reduce the drive current and the heat generated, and to use compact transistors whose speed can be increased. However, with such a conventional configuration, there is a possibility that the load will concentrate in only some of the drive circuits depending on the printing conditions, so that the drive circuits must be set with the assumption that such load concentration will occur. Therefore, there is a tendency for the capacity of the drive circuits to be excessive and for the number of drive circuits installed to be excessive in comparison with the load that actually occurs.
With normal printing, it is rare for all the piezoelectric elements to be simultaneously driven, and the number of piezoelectric elements driven simultaneously is usually half or less of the total number of the elements. This tendency is particularly apparent when inks of some colors are used in a color printer. For example, with a six-color ink printer, the number of piezoelectric elements driven simultaneously is about ⅓ the total number of the piezoelectric elements on average.
However, in a conventional circuit dividing system, the load concentrates in only part of the drive circuits during printing in which the load on a nozzle group is severely increased, such as printing in which only one of the ink colors is used. The waveform distortion and the ink ejection conditions are different in drive circuits in which the load excessively concentrates and in drive circuits there is no concentration. This results in the possibility that image unevenness will occur.
Tests have been conducted as means for resolving these problems, wherein load nonuniformities between circuits are suppressed and waveform distortion in circuit units is reduced by driving, separately from the actual load, a dummy element such as a ceramic condenser instead of a nonoperating piezoelectric element. However, this conventional method of using a dummy element consumes an unnecessary amount of electricity because all the driving wave generating units are operated even when only a small number of nozzles are used.
Therefore, other methods have been considered for arbitrarily switching the connection among a plurality of driving wave circuits and a plurality of piezoelectric element groups, into which the piezoelectric elements are divided, according to the ejection conditions with analog switches or the like (see Japanese Patent Application Publication Nos. 2001-293856 and 2002-103617). It is thereby possible to suitably use the plurality of drive circuits according to the conditions of the piezoelectric element groups. For example, if some groups have a heavy load and some groups have a light load, the drive circuits can be used so that the load is equalized with the multiplexor circuit or the like of the analog switches.
Otherwise, if the load is extremely light, the amount of electricity consumed by the drive circuits can be suppressed by using part of the plurality of drive circuits and not using the remainder of the drive circuits.
According to the methods proposed in Japanese Patent Application Publication Nos. 2001-293856 and 2002-103617, the drive circuits can certainly be distributed and the circuits can be suitably used according to the driving conditions; therefore, the electricity consumed and the heat generated by the circuits can be suppressed; however, the increase in instantaneous current consumption and the power source capacity of the entire system become problematic depending on the application timing of the driving waves. More specifically, when the driving waves are outputted from the plurality of drive circuits with the same timing, the instantaneous electric current increases as seen from the power source (i.e., looking over the system as a whole), and a sufficiently large power source capacity must be prepared even if the drive circuits themselves are divided and the load on each drive circuit is reduced.
Moreover, a voltage drop cannot be avoided even if the power source capacity is sufficiently increased, because of resistance of the wiring in and around the recording head. It is then possible that the drive energy will be insufficient, ink ejection will be unstable, and the recorded images will be unsatisfactory.
In order to avoid such problems, in one method, the number of nozzles that simultaneously perform ejection is analyzed and calculated on the basis of the image data by a CPU or an image processing ASIC (application-specific integrated circuit), and when the calculated number of nozzles exceeds a specific value, the ejection operation for the exceeding portion is stopped, or is postponed until the next ejection operation (see Japanese Patent Application Publication No. 2002-283556). In another method, the ejection drivers are operated through switch ICs (integrated circuits), and the number of the switches turned on, the electric current flowing through the switch ICs, the temperature, and other such factors are electrically determined by the switch ICs or the like, and ejection is forcibly stopped if a certain condition is exceeded (see Japanese Patent Application Publication No. 2003-291342).
Another method considered for resolving the problems described above with instantaneous current consumption and the power source capacity of the entire system is to divide the multiple nozzles into a plurality of blocks and to perform time-divided driving of driving each block with a separate timing. The instantaneous current consumption is suppressed and brought closer to the average current consumption as a result of driving at separate timings, which makes it possible to reduce the capacity of the power source.
However, when an extremely large number of piezoelectric elements are driven individually with separate timings, the rate of printing decreases, throughput is reduced, although a large throughput is a merit of the line-type recording head, and the properties of the printer are degraded as a result.